The study of constraint in biology investigates why organisms or parts of organisms are sometimes apparently resistant to evolutionary change. Constraint has played an important role in the development of such ideas as homology and body plans.

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Constraint, broadly considered, is so general as to be an almost empty concept; any aspect of an organism that has not changed over a certain period of time could be considered to provide evidence for "constraint" of some sort. In order to make the concept more useful, then, it is necessary to divide it into smaller units. First, one can consider the pattern of constraint as evidenced by phylogenetic analysis. However, it is not clear that mere documentation of lack of change in a particular character is good evidence for constraint in the sense of the character being unable to change. It has often been considered more fruitful, then, to consider constraint in its causal sense; what are the causes of lack of change?

The most common explanation for biological constraint is that stabilising selection acts on an organism to prevent it changing, for example, so that it can continue to function in a tightly-defined niche. This may be considered to be a form of external constraint, in the sense that the organism is constrained not by its makeup or genetics, but by its environment. The implication would be that if the organism were placed in a different environment, its previously constrained features would potentially begin to evolve.

Related to the idea of stabilising selection is that of the requirement that organisms function adequately in their environment. Thus, where stabilising selection acts because of the particular niche that is occupied, mechanical and physico-chemical constraints act in a more general manner. For example, the acceleration due to gravity places constraints on bone density and strength for a particular sized animal. Similarly, the properties of water mean that tissues must have certain osmotic properties in order to function properly.
Functional coupling takes the idea that organisms are integrated networks of functional interactions (for example, the vertebral column of vertebrates is involved in the muscle, nerve and vascular systems as well as providing support and flexibility) and therefore cannot be randomly altered without causing severe functional disruption. As Rupert Riedl pointed out, this degree of functional constraint - or burden - generally varies according to position in the organism. Structures literally in the centre of the organism - such as the vertebral column - are often more burdened than those at the periphery, such as hair or toes.

This class of constraint depends on certain types of phenotype not being produced by the genotype (compare stabilising selection, where there is no constraint on what is produced, but rather on what is naturally selected). For example, for a highly homozygous organism, the degree of observed phenotypic variability in its descendents would be lower than those of a heterozygous one. SImilarly, developmental systems may be highly canalized, to prevent the generation of certain types of variation.

It is clear from the above that, although separate, the types of constraints discussed are nevertheless relatable to each other. In particular, stabilising selection, mechanical and physical constraints might lead through time to developmental integration and canalization However, without any clear idea of any of these mechanisms, deducing them from mere patterns of stasis as deduced from phylogeny or the fossil record remains problematic.